An importance of an electricity storage device such as a lithium ion battery, a nickel-metal hydride battery, and other types of secondary batteries (rechargeable batteries) is increasing as an onboard power source for vehicles, and a power source for computers and portable terminals. Especially, the lithium ion battery is expected to be used as high-output power source to be installed in vehicles. The lithium ion battery can provide a high energy concentration while being light weight. As a typical structure of a secondary battery, a battery with a sealed structure (sealed battery) in which an electrode assembly and an electrolyte are housed in a casing, and the casing is sealed can be exemplified.
In charging this battery, a current exceeding a normal level may flow in the battery upon when a defect is present in the battery or when a charger functions improperly. As a result, the battery may be brought to be in an overcharged state. In a case where an abnormality such as the overcharged state occurs in the battery, gas may be generated within the sealed casing. A pressure inside the casing (internal pressure) may increase, and the battery (casing) may swell, or the casing may break due to the gas pressure. As a countermeasure addressing such a defect, a battery provided with a current interruption device is proposed. The current interruption device cuts off a current passage between the electrode assembly housed in the casing and an electrode terminal exposed outside of the casing when the internal pressure of the sealed casing rises to a predetermined internal pressure so as to secure safety of the battery (for example, see Patent Document 1 (Japanese Patent Application Publication No. H6-215746), Patent Document 2 (Japanese Patent Application Publication No. H7-254401), Patent Document 3 (Japanese Patent Application Publication No. H9-129214), and Patent Document 4 (Japanese Patent Application Publication No. H10-233199)).
A conventional current interruption device will be described with reference to FIG. 12 and FIG. 13. A current interruption device 50 is attached to a casing of a sealed secondary battery. As shown in FIG. 12, the current interruption device 50 includes a seal-opening cover 52, an inversion plate 53, and a lead attaching plate 54. The seal-opening cover 52 has conductivity, and is connected to the external terminal 51. The inversion plate 53 has conductivity, and is arranged on an inner side of the seal-opening cover 52. The lead attaching plate 54 has conductivity, and is arranged on an inner side of the inversion plate 53. The lead attaching plate 54 is made of metal, and is fixed to a center portion (contact portion) of the inversion plate 53. A ventilation hole 55 is provided in the lead attaching plate 54, and an internal press of a battery is configured to act on the inversion plate 53. An arrow 57 shows a current passage when the battery is in a normal state. When the battery is in the normal state, a current flows through the lead attaching plate 54, the inversion plate 53, and the seal-opening cover 52 and reaches the external terminal 51 serially. Notably, a supporting member 58 supports a periphery of the lead attaching plate 54. The supporting member 58 is made of an insulating material.
When the battery comes to be in a state such as being overcharged, gas is generated inside the sealed casing, and an internal pressure of the casing increases. At this occasion, as shown in FIG. 13, the internal pressure of the casing acts on the inversion plate 53 through the ventilation hole 55 in the lead attaching plate 54. A fixed portion of a center portion (contact portion) 56 of the inversion plate 53 is broken off, and the inversion plate 53 is inverted upward. The current passage 57 is cut oft and the current is thereby interrupted.